QIU Bo-cang, MARTIN Hai HU, WANG Wei-min, LIU Wen-bin, BAI Xue

(1.Research Institute of Tsinghua University in Shenzhen,Shenzhen 518057,China;2.Shenzhen Raybow Optoelectronics Corp,Shenzhen 518055,China;3.Guangdong Provincial Key Laboratory of Optomechatronics,Shenzhen 518057,China)

Abstract: In this paper, a high efficiency and high reliability 915 nm semiconductor laser is designed and fabricated, which is a key component of the fiber lasers. In order to maximize the electro-optic conversion efficiency of the device, a double asymmetric large-cavity waveguide structure is adopted in the design, and the quantum well structure, waveguide structure, doping, and device structure are systematically optimized. Device simulations show that the device′s maximum electro-optical conversion efficiency reaches 67% at an ambient temperature of 25 ℃. The material is grown by Metal Organic Chemical Vapor Deposition(MOCVD), and a laser chip having a light emitting region width of 95 μm and a cavity length of 4.8 mm is prepared. Tests show that the efficiency of the packaged device and other parameter indices have reached the advanced level of similar devices in the world. In the case that the threshold current is 1 A at room temperature, the slope efficiency is 1.18 W/A, the maximum electro-optic conversion efficiency is 66.5%, the output power is 12 W, and the electro-optical conversion efficiency reaches 64.3%. It can be seen that the test results are in good agreement with that of the device theory simulation. After approximately 6 000 hours of long-life accelerated testing, the device power does not attenuate, indicating that the produced high-power 915 nm laser chip has very high reliability.

Key words: semiconductor laser;electro-optical conversion efficiency;brightness;cavity surface catastrophic power

1 Introduction

引 言

The performance of ytterbium-doped fiber laser(YDFL) has improved dramatically over the past 20 years and its output power has increased at an average rate of 170% per year. In 2009, Stile reported that IPG used large-mode single-mode fiber(large mode area:LMA) by IPG to obtain 10 000 watts of output power[1]. Due to the outstanding characteristics of fiber lasers, including output power, beam quality, easy system integration with industrial equipment systems, laser industrial processing equipment with fiber lasers as its core components has been widely used in various industrial manufacturing processes[2-3]. YDFL has a very broad absorption spectrum (approximately 900-977 nm), and its pump source wavelength is usually set at about 915 nm[4]. Although high-power 915 nm semiconductor lasers are quite mature, industrial and academic circles are still working hard to continuously improve the parameters and the performance of the devices, including beam quality, polarization characteristics, output power, and electro-optical conversion efficiency[5]. Muritaetal. from Hamamatsu Photonics Japan reported high-electron-to-optical conversion efficiency of 976 nm(96 μm) laser in 2013. The device has a maximum electro-optic conversion efficiency of 68% (corresponding to an output power of 6.3 W) at a test environment of 20 ℃, and the efficiency is reduced to 61% at an output power of 15 W[6]. Crump et al. from FBI Research Institute Germany reported that the 96 μm-wide laser had a lifetime test of more than 4 000 hours at 20 W output power[7]. Literature[8-9]reported that the efficiency of a 976 nm device at room temperature at 10 W output power reached 65%. In addition to output power and efficiency, device reliability is also a very important design consideration. Although the reliability of the device is related to many factors, including junction temperature, current density, and optical power density, the optical power density has a more direct effect on the reliability of the device, especially the reliability related to the facet failure. In order to improve the reliability of the device operation, several methods are used, such as low power density design[10], non-absorption window facet technology[11]. In this paper, for the requirements of high power and high electro-optic conversion efficiency and high reliability, we adopt the design concept of double asymmetric large optical cavity[7,12-13]and that of low power density, and systematically optimize the device′s material structure, including the waveguide structure, doping profile, device cavity length, facet reflectance,etc. to improve the device efficiency. Tests have shown that a device with a width of 95 μm and a cavity length of 4.8 mm can reliably operate at 12 W with a threshold current of approximately 1 A, a slope efficiency of 1.18 W/A. When the output power is 12 W, the corresponding electro-optic conversion efficiency is 64.3%, while the highest conversion efficiency is 66.5%(corresponding to 8.9 W of output power). The device′s energy conversion efficiency and other major parameter levels have reached the level of international devices of the same type.

掺有稀土元素镱的光纤激光器(YDFL)性能在过去20余年来取得了惊人的提升，其输出功率的提升速率平均每年达到170%，2009年，Stile报告了IPG公司采用大模场单模光纤(large mode area,LMA)获得了一万瓦的输出功率[1]。由于光纤激光的优异特性，包括：输出功率、光束质量、易于和工业设备系统进行系统集成，以光纤激光器为其核心部件的激光工业加工设备已经大规模应用于各种工业制造过程中[2-3]。YDFL的吸收谱极为宽广(大约900～977 nm)，其泵浦源的波长通常选在915 nm左右[4]。虽然高功率915 nm 半导体激光已经相当成熟，然而工业与学术界仍在不断努力，以期持续改进器件的参数性能指标，包括：光束质量、偏振特性、输出功率以及电光转换效率等[5]。2013年，日本Hamamatsu Photonics公司的Morita等人报导了高电光转换效率的976 nm宽条(96 μm)激光，器件在20℃测试环境下，最大电光转换效率为68%(对应于输出功率6.3 W)，在输出功率为15 W时，效率降低到61%[6]。2009年，德国FBI研究所的Crump等人报导了96 μm条宽的激光器在20 W输出功率时寿命测试超过4 000 h[7]。文献[8-9]报导了976 nm器件在室温下10 W输出功率时效率达到65%。除了输出功率与效率，器件的可靠性也是非常重要的设计考虑。尽管器件的可靠性与多种因素有关，包括结温、电流密度以及光功率密度等，但光功率密度对器件的可靠性特别是与腔面失效有关的可靠性影响更为直接。为了改善器件运行的可靠性，人们采用若干方法，如低功率密度设计[10]、无吸收腔面技术等[11]。针对高功率以及高电光转换效率以及高可靠性这一要求，本文采用了双非对称大光腔结构[7,12-13]、低功率密度设计理念，并系统地优化了器件的材料结构，包括波导结构、掺杂分布、器件腔长、腔面反射率等来提高器件效率。……